It is widely known, that conventional vibratory and tumbler separator methods and devices are generally ineffective when applied to certain materials, such as fine and highly cohesive powders, wet and sticky bulk materials, fiber materials and generally, so-called “difficult-to-screen” materials because the sieves used for sieving of such materials are prone to clogging and blinding and materials have a significant tendency to agglomerate. It is also known numerous cleaning devices for sieve separators, such as disclosed in patents: U.S. Pat. Nos. 7,416,085, 6,422,394, 5,398,816, 5,143,222, 4,929,346, 4,122,006, etc. Several technical solutions, which are the most relevant to the present invention, are reviewed bellow.
By way of example, U.S. Pat. No. 4,929,346 to Si-Lin discloses a self-cleaning screen assembly including a main screen put flatly on a coarser support screen and a ball tray carrying plastic rings and rubber balls to provide self-cleaning of the screens by bouncing and tapping of the rings and balls between the tray and support screen when they are subjected to vibratory excitation together with housing of the separator.
The similar self-cleaning sieve assembly is sold by SWECO corporation of Florence, KY, USA under trade name “SWECO Sandwich Screens” (see http://www.sweco.com/products_partscreens_screens_sandwich.html of 2009, Oct. 24). The sieve assembly is comprised of a working mesh screen on top of the tension ring with a coarser support screen attached to the bottom of the ring with sliders and/or balls placed between the meshes. The sliders and balls will bounce off of the support screen and tap the top screen to dislodge near-size particles or fibers that tend to blind the screen and reduce screening area.
Disadvantages of these systems include relatively low energy transfer to the sieve, insufficient de-agglomeration and sifting of “difficult-to-screen” materials.
There are also known separation devices, which employ double frequency excitation. These devices, for example, combine use of a low frequency vibration, typically in the range 5-30 Hz, with ultrasonic excitation at 20-50 kHz, provided by means of an electromechanical transducers fed by an electronic generator, which provide high-frequency vibrations of a sieve. By way of example, U.S. Pat. No. 5,398,816 to Senapati discloses a typical screening system having a resiliently mounted frame with a screen extending thereacross. The frame is vibrated by a low frequency vibratory drive using eccentric weights. About the peripheral frame of the screen, a high frequency drives are employed to vibrate the screen in the range of 20,000 Hz. Disadvantages of this system include relatively low energy transfer to the sieve fabric and insufficient de-agglomeration efficiency. Also the cost of ultrasonic systems is high together with current expenses on frequent replacement of worn and torn fine sieves.
Another dual frequency device is a screen energizer disclosed in U.S. Pat. No. 7,182,206 to Hukki et all. A screening system including a vibratory screen separator having a resiliently mounted frame with a low frequency vibratory drive coupled to that frame. A taut screen is rigidly mounted in the frame and a vibration transmitter assembly is resiliently mounted to the frame and fixed to the taut screen. The vibration transmitter includes a planar ring compressed against the taut screen and vibration generators. The vibration generators are air turbines with eccentric weights. The frame includes support elements extending from the cylindrical outer housing sections of the separator to a concentrically mounted support ring. Compressed air is provided to the turbines through hollow structure within the frame. Valves control exhaust from the turbines. The low frequency vibratory drive operates in a range of about 8 Hz to 30 Hz while the vibration generators provided by the air turbines operate in a range of about 275 Hz to 600 Hz. Among the main disadvantages of this device are relatively narrow band spectrum excitation and a low transference of mechanical energy to the screen, causing low performance, besides, complexity and operation inconvenience due to necessity in additional vibratory source of one more energy type.
Cleaning device having beating elements is disclosed in U.S. Pat. No. 7,416,085 to Kadel. The device is provided with a sieving mat, which has sieving openings, and with beating elements, which are located underneath the sieving mat and which strike against the underside of the sieving mat in order to free the sieving mat from material to be sieved that is clogging the sieving openings. Said beating elements are fastened to at least one elongated tensioned traction mechanism such as a cable or band that extends underneath the sieving mat.
Main disadvantages of beating devices of this and similar kinds, when used for fine screening, are connected with fast wear and tear of the sieve due to a local application of impact pulses, limited zone of cleaning action at the sieve surface and insufficient de-agglomeration of material to be screened.
Self-cleaning separation systems, having efficient screening performance due to generation of multifrequency vibration having a wide band frequency spectrum are disclosed in U.S. Pat. No. 6,845,868 to the Inventors. Typically a multifrequency separator includes a housing, a source of single frequency vibration, and a mechanical converting system, which converts single frequency vibration of housing into a sequence of mechanical pulses applied to an interface apparatus, thereby to generate a multifrequency vibration of the sieve so that to provide de-agglomeration and efficient segregation of the material bed, and to prevent blockage of the sieve. The interface apparatus also protects fine sieve from local impact loads and thus prolongs sieve lifetime between replacements. Multifrequency separating systems, designed as inserts to existing vibratory separators as well as autonomous machines constructed from the outset are proved to provide a stable blind-free separation of numerous difficult-to-screen materials. However, when compared to conventional separators, these systems: have more complicated design, require more precise assembling and tuning, and enlarge overall dimensions of the machine.